This invention relates to optical systems and in particular to an athermalised dual field-of-view objective optical system capable of operating in the 3-5 micron thermal infra-red waveband and which is light in weight.
The major problem in most thermal infra red waveband lens design is athermalisation. In particular much of the thermal focus shift arises due to thermal refractive index change at the first lens. Accordingly prior proposals have either utilised a first lens which has been multiple element (which significantly increases weight because the first lens is diametrically the largest in the system) or single element made of zinc sulphide (which has a thermal change of refractive index coefficient which is close to zero). Zinc sulphide, however, is very expensive, heavy, and has comparatively poor transmission.
It is an object of the present invention to provide a relatively inexpensive athermalised light weight optical system capable of operating in the 3-5 micron waveband and which has a fixed front lens in the form of a single element made of silicon. Silicon does not have a thermal change of refractive index coefficient which is close to zero but is relatively inexpensive, light in weight and has good transmission. Additionally silicon is a non toxic material so that it does not present a health hazard during lens manufacture.
According to the present invention there is provided a dual field-of-view objective optical system capable of operating in the 3-5 micron thermal infra-red waveband, comprising
a fixed front lens and a fixed rear lens aligned on a common optical axis and an intermediate lens located between the front and rear lenses and movable axially along the optical axis between two alternative in-use positions which provide two alternative focal lengths for the system,
said front lens being a positively-powered single element made from silicon which is relatively non-dispersive in the 3-5 micron waveband and relatively insensitive to thermal change of refractive index,
said intermediate lens being negatively powered and comprising a first lens element of greater negative power than is required solely for the field of view change and being made of a material which is relatively dispersive in the 3-5 micron waveband and relatively sensitive to thermal change of refractive index, and a second lens element of positive power and made of a material which is relatively non-dispersive in the 3-5 micron waveband and relatively insensitive to thermal change of refractive index, said rear lens being positively powered and comprising a first lens element which is positively powered and made of a material which is relatively non-dispersive in the 3-5 micron waveband and relatively insensitive to thermal change of refractive index, and a second lens element which is negatively powered and made of a material which is relatively dispersive in the 3-5 micron waveband and relatively sensitive to thermal change of refractive index,
wherein each lens element in the system is made of a material having a positive temperature/refractive index change coefficient and the relative power of the lens elements of the intermediate and rear lenses are arranged simultaneously to provide correction for chromatic aberration and thermal defocus in both focal settings of the system.
Preferably said single element fixed front lens incorporates an aspheric refractive surface to provide correction for monochromatic aberration. Since the front lens is a single element it is not itself corrected for chromatic aberration and therefore the system has an imperfect but extremely good degree of chromatic aberration correction. A further improvement in chromatic aberration correction can be achieved by imposing a hybrid diffractive surface onto the aspheric surface of the first lens. This however, increases manufacturing costs and reduces transmission.
Preferably said rear lens is formed by three lens elements and the third lens element is positively powered and made of a material which is relatively non-dispersive in the 3-5 micron waveband and relatively insensitive to thermal change of refractive index. This arrangement of the rear lens having two positively powered lens elements enables these lens elements to be made of different materials which assists with correction of monochromatic aberration (according to their relative powers) and correction of residual errors in thermal and chromatic aberration (according to their relative dispersive characteristics).
Preferably the negatively-powered lens elements of the intermediate and rear lenses are made of Germanium. The positively-powered lens elements of the intermediate and rear lenses may be made of Silicon and/or Zinc Sulphide and/or Zinc Selenide and/or Arsenic Trisulphide.
By virtue of the present invention a dual field-of-view objective system is provided which is relatively inexpensive to manufacture and which can be made with high transmission whilst being essentially achromatised and athermalised. One example avoids use of surface-relief hologram surfaces diamond cut onto the front lens, such hybrid surfaces being known to reduce transmission and being a manufacturing on-cost. This example however is not fully achromatised for off-axis field angles, suffering from a small residual amount of transverse chromatic aberration. Another example, which is fully achromatised on and off axis, incorporates a surface-relief hologram on the front lens.